# 反转一个单链表。

# 示例:

# 输入: 1->2->3->4->5->NULL
# 输出: 5->4->3->2->1->NULL

# Definition for singly-linked list.


class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None or head.next is None:
            return head
        pre = None
        cur = head
        h = head
        while cur:
            h = cur
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return h


if __name__ == '__main__':
    s = Solution()
   
